Working principle, process flow and inspection method of blueing (blackening) treatment on steel surface
Steel products are blueed (blackened) for corrosion resistance. So why do we need to do it? What is the working principle of blueing (blackening)? What is the process flow? What is the inspection method? This article introduces it in detail.

1. Blueing (blackening) principle

In order to improve the rust resistance of steel parts, a strong oxidant is used to oxidize the surface of steel parts into dense and smooth ferroferric oxide (Fe3O4). This thin layer of Fe3O4 can effectively protect the inside of the steel parts from oxidation. Fe3O4 oxidized at high temperature (about 550℃) is sky blue, which is called blueing treatment. Fe3O4 oxidized at low temperature (about 130℃) is dark black, which is called blackening treatment. In the weapons industry, blueing treatment is commonly used; in industrial production, blackening treatment is commonly used.

Whether iron can be oxidized into dense and smooth Fe3O4 depends on the selection of a strong oxidant. Strong oxidants are composed of sodium hydroxide (NaOH), sodium nitrite (NaNO2), and trisodium phosphate (Na3PO4). When bluing, their molten liquid is used to treat the workpiece; when blackening, their aqueous solution is used to treat the workpiece. The concentration of this solution is 1.4g/cm3, and the boiling point is 130℃. Under the action of this solution, the oxidation process of iron is as follows: Fe→Ma2FeO2→Na2Fe2O4→Fe3O4

Production practice experience has shown that in order to obtain a bright and dense Fe3O4 film, the ratio of sodium nitrite to sodium hydroxide in the oxidation solution should be maintained between 1:3 and 3.5.

2. The operation process of bluing (blackening):

Workpiece clamping → degreasing → cleaning → pickling → oxidation → cleaning → saponification → hot water boiling → inspection.

(I) Workpiece clamping Special fixtures or hangers should be designed according to the shape and size of the workpiece. The purpose is to leave enough gaps between the workpieces, so that the workpieces cannot touch each other, and each workpiece can be completely immersed in the oxidizing solution for oxidation.

(II) Degreasing The purpose is to remove the oil stains on the surface of the workpiece. After the rack work (blueing and blackening are the last process), the surface of the workpiece will inevitably leave oil stains, especially when anti-rust oil is used for rust prevention between processes. Any oil stains will seriously affect the formation of Fe3O4, so they must be removed before blueing and blackening. Heat the degreasing solution to 80-90℃, and then immerse the workpiece for about 30 minutes. If there is a lot of oil stains, the degreasing time must be extended to ensure that the oil is completely removed.

(III) Pickling

The purpose of pickling is to remove rust on the surface of the workpiece. Because rust will hinder the formation of a dense Fe3O4 layer. Even if the workpiece has no rust, it should be pickled because it further removes the oil stains, and pickling will increase the activation energy of the molecules on the surface of the workpiece, which will be beneficial to the oxidation of the next process and can form a thicker Fe3O4 layer. The pickling solution is generally a 10%-15% concentration of sulfuric acid solution at a temperature of 70-80℃. , immerse the workpiece in sulfuric acid solution for about 30 minutes. The workpiece with less rust can be immersed for 20 minutes, and the workpiece with more rust needs to be immersed for more than 40 minutes.

(IV) Oxidation

Oxidation is the main process of bluing and blackening. Whether the Fe3O4 film is dense, smooth, and thick enough depends on the oxidation stage. The bluing temperature is 550℃. The temperature of the blackening solution is 130-145℃. The immersion time is 50-80min. High-carbon steel with high carbon content has a faster oxidation rate and the immersion time can be shorter. Low-carbon steel with low carbon content has a slow oxidation rate and needs a longer immersion time. Alloy steel, especially high-alloy steel, has a certain amount of residual austenite on the surface of the workpiece, which has a strong resistance to alkaline solution and is not easy to generate Fe3O4, so the immersion time needs to be longer. The normal color of the solution is white. If the solution is red or brown, it means that the concentration of sodium nitrite is too high, and the composition of the solution should be adjusted in time. If the solution is green, it means that the iron ion concentration is too high and the solution should be updated in time.

(V) Saponification

Saponification is to soak the workpiece in a soapy water solution at a certain temperature. The purpose is to form a layer of iron stearate film to improve the corrosion resistance of the workpiece. The commonly used saponification solution concentration is 30-50g (soap)/L. Heat the saponification solution to 80-10℃ and soak the oxidized workpiece for about 10 minutes.

(VI) Inspection

After oxidation, the workpiece should be inspected to see if the Fe3O4 layer is qualified. Three pieces are randomly selected and soaked in a 2% copper sulfate solution for 20 seconds. Those that do not fade to black are qualified. If more than one of the three pieces is unqualified, the entire tank workpiece is considered unqualified and needs to be oxidized again to deepen the Fe3O4 thickness.

Through the above bluing treatment (blackening treatment), the steel surface achieves corrosion resistance and surface modification. This corrosion resistance is generally resistant to atmospheric corrosion, and it is best to have anti-rust oil on the surface.